Combustion gases, and especially flue gases from gas turbines often comprise a substantial quantity of carbon dioxide, which is a known greenhouse gas. Thus, isolation and/or sequestration of carbon dioxide from combustion processes has gained significant attention over the last decade, and there are numerous configurations and methods known in the art to remove carbon dioxide from a flue gas.
For example, carbon dioxide may be removed from various gas streams with one or more membranes as described in U.S. Pat. No. 4,130,403 to Cooley et. al., U.S. Pat. No. 4,639,257 to Duckett et. al., or U.S. Pat. No. 5,233,837 to Callahan. Membrane processes typically exhibit relatively high selectivity towards a particular gas component. Moreover, membrane processes can generally be operated without energy consuming circulation (e.g, heating and/or cooling requirements that are often needed for solvent based carbon dioxide removal). However, and especially depending on the feed gas composition, membrane life time is less than desirable, or the feed gas requires pretreatment before contacting the membrane. Furthermore, membrane systems typically operate at a relatively high pressure differential, which either necessitates a blower or other pressure increasing equipment for low pressure feed gases or disqualifies membrane systems for such low pressure feed gases.
Alternatively, carbon dioxide may be removed using physical or chemical solvents, and numerous process configurations for solvents are known in the art. Physical solvent processes are particularly advantageous where the acid gas partial pressure in the feed gas is relatively high. Thus, all, or almost all physical solvents exhibit only limited usefulness for the removal of carbon dioxide from flue gases which are typically near atmospheric pressure, and especially where the flue gas has a relatively low carbon dioxide content.
To circumvent problems associated with the use of physical solvents, chemical solvents may be employed to scrub the feed gas, wherein the chemical solvent is regenerated downstream to recover the carbon dioxide. Scrubbing gases with chemical solvents typically allows removal of carbon dioxide from a feed gas at relatively low pressure. However, such methods are often energy intensive as well as costly and problems with corrosion and solvent degradation frequently arise (see e.g., U.S. Pat. No. 2,065,112, U.S. Pat. No. 2,399,142, U.S. Pat. No. 2,377,966, U.S. Pat. No. 4,477,419, or U.S. Pat. No. 3,137,654). Moreover, as the carbon dioxide partial pressure in the feed gas decreases (e.g., the exhaust gas from a gas turbine operated with relatively large amount of excess air, as well as that from a HAT cycle), the size of the recovery equipment as well as the power consumed by a blower typically increases substantially to overcome the pressure drop in the recovery equipment.
Thus, although various carbon dioxide removal configurations and processes are known in the art, all or almost all of them suffer from one or more disadvantages, especially where the partial pressure and/or concentration of the carbon dioxide in the feed gas is relatively low. Therefore, there is still a need to provide improved configurations and methods for carbon dioxide recovery from various gases, and especially gases with relatively low carbon dioxide partial pressure.